This invention relates to a method for treating the primary and secondary bitumen froth streams produced from bituminous sands by a hot water extraction process plant. More particularly, it relates to a system for heating and deaerating bitumen froth prior to introducing it into a system for recovery of the bitumen from the deaerated stream.
One of the world's largest reserves of hydrocarbons is contained in the tar sands deposits located in Northern Alberta. Bitumen is presently being recovered from this tar sand using the known hot water extraction process.
In general terms, this process involves mixing tar sand with water, steam and sodium hydroxide in a rotating tumbler to initially disperse the bitumen. A typical slurry issuing from the tumbler comprises 9.5% bitumen, 68.5% solids and 22.0% water (all percents being by weight). The slurry has a temperature of about 180.degree. F and a pH of 8.0-9.5. Hot water is added to the slurry to dilute it and provide a stream typically containing 7% bitumen, 50% solids and 43% water. This diluted slurry is then introduced into a cylindrical vessel having a conical bottom. The bulk of the contained coarse sand particles settles out in this vessel and is removed as an underflow and discarded. The more buoyant bitumen particles float to the surface of the vessel to form a froth comprised of bitumen plus entrapped water and solids. This froth overflows the vessel wall and is received in a launder extending around its rim. The product is commonly called "primary froth" and typically comprises 66.4% bitumen, 8.9% solids and 24.7% water. It is usually at a temperature of about 175.degree. F. A middlings stream, typically comprising 2.3% bitumen, 21% fine solids (i.e. -325 mesh) and 76.8% water, is withdrawn from the mid-section of the settler and is pumped to a sub-aeration flotation cell. In the cell, the middlings are vigorously agitated and aerated. The middlings bitumen and some water and solids float to the surface and form "secondary froth." This froth typically compresses 41.4% bitumen, 12.4% solids and 46.2% water. It is recovered in a launder and is then retained in a tank to settle out some water and solids. The settled product typically comprises 41.4% bitumen, 12.4% solids and 46.2% water and has a temperature of 170.degree. F. Heretofore the primary froth and settled secondary froth have been combined and pumped to a heating and deaerating column. In this column the viscosity of the bitumen is reduced from about 1500 centipoise at 160.degree. F. to about 500 centipoise at 190.degree. F. The hot deaerated product is then diluted with naphtha and introduced into a centrifuging system to recover clean dilute bitumen.